Biochemistry and clinical relevance of lipoprotein X

Lipoprotein X (LP-X) is an abnormal lipoprotein that appears in the sera of patients with obstructive jaundice and is thus a marker for cholestasis. The presence of LP-X in serum does not allow discrimination between intra- and extra-hepatic cholestasis. In addition LP-X is present in the plasma of patients with familial plasma lecithin: cholesterol acyl transferase (LCAT) deficiency. It is a spherical particle that aggregates strongly. Phospholipids and unesterified cholesterol make up the bulk of LP-X, which is a low density lipoprotein. Protein, cholesterol esters, and triglycerides together make up 12 percent of the composition of LP-X. Lithocholic acid is the major bile acid in LP-X. Three species of LP-X have been isolated (LP-X1, LP-X2 and LP-X3). Because of its aggregating properties, LP-X complexes with enzymes, such as alkaline phosphatase. Electrophoretic and immunochemical methods are available for assay of LP-X. The fact that bile lipoprotein can be converted to LP-X by addition of albumin, and LP-X can be converted to bile lipoprotein by the addition of bile salts may suggest that the integrity of the LPX molecule depends on a certain critical bile salts to albumin ratio. Phospholipase in plasma is implicated in the catabolism of LP-X. The role of LP-X in cholestasis is apparently related to the removal of free cholesterol from the circulation as a consequence of its aggregating properties.